1. Field of the Invention
The present invention concerns polyester gas separation membranes which are obtained by the interfacial polymerization (IFP) of benzenetricarbonyl trichloride or benzenetetracarbonyl tetrachloride in a water-insoluble organic solvent in one phase and a variety of difunctional phenols present as the di-alkali metal salt in a mixture of water and a phase transfer catalyst and a water-soluble organic solvent in the second phase. The present invention also includes the polyester membrane itself, its methods of preparation and the membranes as used to separate gas mixtures, such as carbon dioxide and methane, helium and methane and the like.
2. Description of the Related Art
Any commercial gas membrane must be strong and have useful transport properties. In composite membranes, the functions of strength and attractive transport properties are optimized by incorporating two separate polymer layers--a porous support covered by a continuous non-porous discriminating layer. The support provides the strength, but provides little or no separation power or resistance to gas flow. The discriminating layer provides useful transport properties and virtually all of the resistance to flow. The discriminating layer must be thin to achieve the maximum transport rate. Its thickness is limited by the size of the pores in the support. The discriminating layer must be thick enough and strong enough so that when deposited over the largest pore in the support and subjected to an applied pressure, it resists breaking. Obviously, the polymer that makes up the discriminating layer must have good film forming properties, which usually is associated with chain entanglement or crosslinking. Composite membranes are often made by the IFP method. For example, reverse osmosis (RO) membranes are prepared by a process in which an aqueous solution of metaphenylenediamine or another multifunctional polyamine is put into the pores of a membrane support. The 1,3,5-trimesoyl chloride (TMC) solution is washed over the wet support. If done correctly, a thin film of crosslinked polymer is produced on the support. The IFP method produces polymer with good film forming properties because the film is crosslinked. The thickness of the discriminating layer is limited because the polymerization slows after the water/organic interface is sealed. This results in thin, strong, leak-free composite membranes.
Interfacial polymerized (IFP) membranes are useful in fluid separations. FilmTec, a Dow Chemical Company subsidiary in Minneapolis, Minn. currently makes water purification membranes using the IFP method. The FilmTec membranes have been optimized for liquid water purification, but presently do not have industrially interesting gas separation or transport properties. However, there may be advantages of the IFP method which are beneficial for membranes used in gas separations. No one thus far has reported the polyester membrane of the present invention for gas separations.
FT30 is a commercial reverse osmosis (RO) membrane manufactured by FilmTec. It is made through the interfacial polymerization of meta-phenylene diamine (MPD) with 1,3,5-benzenetricarbonyl trichloride (TMC for trimesoyl chloride) (see U.S. Pat. No. 4,277,344) producing a crosslinked polyamide. The reverse osmosis (RO) membrane, FT30 (polyamide), when tested for gas separations has extremely high transport rates and low selectivities. FT30 gas transport measurements show water vapor flux of greater than 3.0*10.sup.-03 cc/cm.sup.2* sec* cmHg which is the limitation of the gas testing apparatus, with little or no selectivity for other gases present or tested. The permeance of the other gases (which are normally faster than water) were not determined because the water overwhelmed the equipment. The selectivity was not determined. The crosslinking and the stiffness of the polymer chains in FT30 probably produce many gas transport channels that are too large to discriminate between gas molecules. Very flexible polymers produce membranes with either low flux or poor selectivity i.e. CO.sub.2 /CH.sub.4 selectivity&lt;30. Therefore, polymer flexibility must be modulated carefully.
The following art is of general and specific interest.
J. N. Anand et al., in U.S. Pat. No. 4,840,646 disclose a method of preparing a linear polyestercarbonate wherein the diphenolic portion is a tetrabromodiphenol residue. The membranes produced by these polymers have good mechanical properties and are useful in gas separations.
T. O. Jeanes, in U.S. Pat. No. 4,851,014 discloses the preparation of linear polycarbonates, polyesters and polyestercarbonates containing tetrafluorobisphenol-F. The membranes of these polymers are useful in gas separations.
J. H. Kawakami et al., in U.S. Pat. No. 9,994,095 disclose the preparation of a defined linear polyester of 4,4'-(IH-alkylidene)-bis[2,3,6-trialkylphenol] and aromatic acids. The membranes of these polymers are useful in gas separations.
F. Ueda et al., in U.S. Pat. No. 4,493,714 teach the preparation of ultrathin film and its use for concentrating a specified gas in a gas mixture. Polyamines composed of silicon-containing polyamines are combined within polyisocyantes.
W. C. Babcock et al., in U.S. Pat. No. 4,781,733 teach the preparation and use of semi-permeable thin-film membranes of siloxane, alkoxysilyl and aryloxysilyl oligomers and copolymers. Some of the polymers for reverse osmosis (RO) are produced using interfacial polymerization.
J. K. Nelson, in U.S. Pat. No. 4,822,382 discloses the preparation of composite membranes and their use in the separation of gases. The composite membrane has a separation layer of difunctional poly(tetramethyl)bisphenol A phthalate.
R. P. Castro et al., in U.S. Pat. No. 5,049,167 disclose the formation of polyamide membranes using interfacial polymerization. The polyamine monomers are reacted with toluene-2,4-diisocyanate or 1,3,5-benzene tricarbonyl trichloride. No polyalcohol monomers are described or suggested to produce polyesters. These polyamine polymers are useful as membranes for the separation of gases.
G. Sartori et al., in U.S. Pat. No. 5,177,296 teach the preparation of linear polyesters and the thermal crosslinking of the membranes formed from the polymers. The membranes are used to separate aromatic organic compounds from saturated organic compounds.
M. M. Chou, et al. in U.S. Pat. No. 5,271,843 disclose a semipermeable membrane which is useful for reverse osmosis. The membrane is a polyamide produced by the interfacial polymerization on a support of a polyamine and an aromatic polyacyl halide.
J. E. Tomasche in U.S. Pat. No. 5,246,587 discloses a water permeable reverse osmosis membrane. The membrane is a polyamide formed by interfacial polymerization on a support of a polyamine and an aromatic polyacyl halide.
None of the art cited herein individually or in combination teach or suggest the present invention.
It is apparent that a need exists for improved polyester membranes for improved separation of mixtures of gases. Difunctional phenol chemistry is attractive because many monomers of varying rigidity were available. Furthermore, S. Bales et al., U.S. Pat. No. 4,840,646 show that non-crosslinked polyesters based on biphenols generally have interesting selectivities in the separation of gases.
In the present invention, (1) crosslinked polyester membranes having glassy polymer characteristics have both a high selectivity for gas separations and high flux; (2) high selectivity demonstrated by these novel compositions is surprising and unexpected because all prior art using rubbery polymers inherently has low selectivities; (3) these novel glassy membranes have the added advantages of being resistant to flux losses at high pressures and at elevated temperatures; (4) highly crosslinked glassy polymer membranes made by interfacial polymerization generally or previously yielded membranes with substantially lower selectivity than the linear polymers; and (5) crosslinking of linear polymers produces large flux losses which are a disadvantage.
The present invention provides the improvements in membranes for separation of gases as is described herein below.